You can get a turbo back exhaust either with no cats or with a high flow cat. Neither are legal. According to the law, you cannot remove or replace a catalytic converter unless it is malfunctioning and you replace it with a direct replacement. Of course, the same goes for mufflers, which makes cat-backs illegal.

As for your second question, there is no such thing as "better" when talking about uppipes and downpipes. They serve different purposes. An uppipe protects your turbo when you start raising boost or changing your AFR because it gets rid of the first cat. The downpipe makes more power.